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Can AC coexist with DC?

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    #1

    Can AC coexist with DC?

    This is probably a stupid question to which I already know the answer based on my knowledge but I'll ask any way.

    Long story short, this is before my time and experience with electronics. My dad (and now I) are designing a hard-wired doorbell/strobe system for the house, but the project had been shelved for several years due to time and design issues until now.

    He bought a board off ebay several years ago, that can take 24VAC and step it down and invert it to 12VDC, and it does. Measuring the output I get 12VDC but also get 24VAC, but measuring the input I get no DC but the 24VAC.

    It obviously works since the strobes do work, they didn't before with straight AC and they specify 12 VDC on it's label which is why my dad bought this board.

    It does have a linear LT1074
    https://cdn.badcaps-static.com/pdfs/...e6e1442403.pdf

    Can AC coexist with DC?

    Should I be concern?
    Last edited by Mad_Professor; 09-08-2014, 08:03 PM.
    Tags: None
    #2
    Re: Can AC coexist with DC?

    cable companies and telco companies do it all the time. centurylink has 24v dc running along with voice/dsl data.
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      #3
      Re: Can AC coexist with DC?

      Can we see this board and info about it such as spec of the input, output rating, etc?
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        #4
        Re: Can AC coexist with DC?

        What do you mean by coexist... It is possible to superimpose an AC signal over a DC signal but that waveform pretty much is AC for all intents and purposes, though it never goes negative - i.e., you can't / don't need to rectify it. It's no longer DC really anymore as you'll need a capacitor to filter out the AC, but again you don't need to rectify it.

        If you mean time division multiplexing the AC/DC so that one or the other is on the wire at any one time, that's also possible but you still have to design the circuit to tell the difference and not fry itself in the process. Like phone lines as said, normally 48VDC is sent along the wire, but when it comes time to ring the telephone bell, a 90VAC signal is sent. The higher, alternating current will ring the bell but the 48V will not, and the 90VAC won't let you transmit voice over it...

        Then the DSL signal is another beast, it's frequency division multiplexed on the wire...

        Comment

          #5
          Re: Can AC coexist with DC?

          Originally posted by budm View Post
          Can we see this board and info about it such as spec of the input, output rating, etc?
          I have photos, but I don't think you're going to find any information on it. but at least you'll see how the circuit is. I didn't want to remove it from its platform since my dad worked hard on it.

          Originally posted by eccerr0r View Post
          What do you mean by coexist...
          What I mean is I'm getting 24v AC and 12 DC on the output of this inverter board, when the input is only 24v AC, which I want to invert to 12v dc to run the strobes. Considering I have a 24v AC door bell transformer to pull power from, and the relays I have are 24v AC as well, but the strobes are 12v DC hence the need to invert.

          But I shouldn't be getting 24v AC and 12v DC on the output.
          Attached Files

          Comment

            #6
            Re: Can AC coexist with DC?

            Still doesn't make sense. When you "invert" DC you get AC. Inverting AC gets you AC.

            As a doorbell system tends to be a transformer, door bell button, and the bell, I'm not sure why you can't connect the strobe parallel to the bell and have the 12VDC output separate from the 24VAC system. Or was your intent to somehow put the 12VDC transformer near the pushbutton switch and have the 12VDC share the 24VAC line to the doorbell? This requires some forethought to get it to work.

            Perhaps an accurate drawing of what you want including distances would be helpful.

            Comment

              #7
              Re: Can AC coexist with DC?

              May I ask what kind of multimeter you are using to measure this?

              The reason I ask is that many less-expensive multimeters use DC coupling for the AC voltage ranges (ie. no DC-blocking input cap.) This causes the meter to give a non-zero reading for any positive DC voltage applied while the meter is set for ACV. Due to the scaling used for AC, the reading will be about twice the DC input. Negative DC voltage will give a zero ACV reading however, because the meters use a single diode as a half-wave rectifier for AC.

              Comment

                #8
                Re: Can AC coexist with DC?

                Originally posted by eccerr0r View Post
                Still doesn't make sense. When you "invert" DC you get AC. Inverting AC gets you AC.

                As a doorbell system tends to be a transformer, door bell button, and the bell, I'm not sure why you can't connect the strobe parallel to the bell and have the 12VDC output separate from the 24VAC system. Or was your intent to somehow put the 12VDC transformer near the pushbutton switch and have the 12VDC share the 24VAC line to the doorbell? This requires some forethought to get it to work.

                Perhaps an accurate drawing of what you want including distances would be helpful.
                ok, convert whatever the term is. (please do correct me on this.)

                The problem was the strobes needed to run for about 10 seconds after the bell is rang. The strobes didn't work with 24VAC, even when you held the button down, also the strobes don't work with my power brick which puts out 24v AC. They only work with DC.

                I have two of these (Chime bells)
                http://www.amazon.com/Heath-Zenith-1.../dp/B000BQMKU6
                in series which I plan to change to parallel due to the fact they don't sound right, they make more of a buzzer type sound instead of ring.

                The idea is when a person rings the door bell, it triggers a solid state relay. Why solid state? due to the fact that the bell has inconsistent voltage for a regular relay to engage. This solid state relay triggers a timed relay to stay closed for 10-15 seconds before going open again. This is where DC power and strobes are connected.

                I don't intend on putting DC on AC, I've already ran the bell wire, Right now it's one AC and one DC out to the front, going to add another run to make the button on it's own circuit or I might do it differently, then for the back chime bell it's just AC.

                What I am getting at is that I'm getting a reading that I shouldn't and modemhead might be right it might be my DMM.

                Originally posted by modemhead View Post
                May I ask what kind of multimeter you are using to measure this?

                The reason I ask is that many less-expensive multimeters use DC coupling for the AC voltage ranges (ie. no DC-blocking input cap.) This causes the meter to give a non-zero reading for any positive DC voltage applied while the meter is set for ACV. Due to the scaling used for AC, the reading will be about twice the DC input. Negative DC voltage will give a zero ACV reading however, because the meters use a single diode as a half-wave rectifier for AC.
                A piece of shit cen-tech. I did have a fluke, but it got stolen along with few other tools, acouple years back.
                Last edited by Mad_Professor; 09-09-2014, 01:02 PM.

                Comment

                  #9
                  Re: Can AC coexist with DC?

                  I have a cheap GB Instuments meter that will read 3v from a 1.5v D cell on AC setting, however polarity doesn't matter to this meter so it must have a full-wave rectifier.

                  My better Mastech meter doesn't do that. It'll show a small voltage that quickly goes down as the DC de-coupling cap charges.

                  Comment

                    #10
                    Re: Can AC coexist with DC?

                    Originally posted by Mad_Professor View Post
                    What I am getting at is that I'm getting a reading that I shouldn't and modemhead might be right it might be my DMM.
                    Measure a 9V battery with your DMM set to ACV, red to the positive battery terminal, black to the negative. If it reads 18V or so, it's the meter. If you switch the leads around, it should read zero.
                    Attached Files

                    Comment

                      #11
                      Re: Can AC coexist with DC?

                      Originally posted by modemhead View Post
                      Measure a 9V battery with your DMM set to ACV, red to the positive battery terminal, black to the negative. If it reads 18V or so, it's the meter. If you switch the leads around, it should read zero.
                      Yep it was the meter.

                      I have another meter called Excel that was gifted to me by badcaps member mariushm, and it reads 0 on AC. I retested the board it reads 0 now. So I guess it's the crappy cen-tech.

                      Here a video of the setup with commentary.
                      http://youtu.be/pHfKMTxQvME

                      Comment

                        #12
                        Re: Can AC coexist with DC?

                        Now what does a True RMS meter measure with a DC voltage...

                        Comment

                          #13
                          Re: Can AC coexist with DC?

                          Originally posted by Mad_Professor View Post
                          Yep it was the meter.

                          I have another meter called Excel that was gifted to me by badcaps member mariushm, and it reads 0 on AC. I retested the board it reads 0 now. So I guess it's the crappy cen-tech.

                          Here a video of the setup with commentary.
                          http://youtu.be/pHfKMTxQvME
                          Looks good. So the "Simple Switcher" 12V output feeds the delay, which then feeds the strobe for the fixed period.

                          If you need more brightness, you can run the output of the time delay into an astable 555, repeating at 3-5 Hz. Then, use the 555's output to drive a large relay connected to a 100-150W incandescent lamp. A coil of modest drive requirements can be directly driven by the 555.

                          And, to make it interesting, you can even vary the duty cycle of that astable, to make a more striking display- long on time and short off time, for example.

                          See the 830 schematic. It's obvious why/how those meters read DC as "AC" with the leads one way.

                          Attached Files
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